This invention relates to an inexpensive, low power synchronous inductor motor and, in particular, to a low power synchronous inductor motor for driving a clock movement.
In the past there have been two general types of synchronous motors used in the industry to drive electric clock movements, namely, the hysteresis motor and the inductor motor. The earliest electric clock movements turned out in great volume employed a 3600 rpm hysteresis motor. The two stator poles of this motor were split to accommodate large copper shading rings which caused the pulsating magnetic field provided by the stator energizing winding to be converted to a rotating field which is required for unidirectional and self-starting rotation of the roter. The high rotor speed required a special finish on the shaft and bearings of the motor and on the gear train. Further, precision cutting of the gear train components, complete encapsulation of the rotor and gear train and careful attention to the internal lubrication of the rotor and gear train was required. With each of the aforementioned prerequistes, the motor was still subject to a short life and was inherently noisy.
Later another clock movement manufactured in great volume used a 900 rpm, 8 pole hysteresis motor. Because of the low rotor speed, the movement was quieter and had a longer life than the aforementioned high speed design. However, this movement also required an encapsulated rotor and gear train and in addition required elaborate precautions regarding the lubrication and the quality of parts.
Another prior art motor produced in great volume was an inductor motor that had a rotor which rotated at 200 rpm. The rotor structure was made up of a permanent magnet disc with the outer peripheral edges of the disc magnetized alternately with north and south poles. With this arrangement the flux generated by the stator winding passed through the high reluctance path of the permanent magnet thereby causing substantial power and heating loses in the rotor.
Generally, prior art clock motors operated at a developed torque which was substantially more than required to rotate the hands of a clock. Typically, these motors dissipated two watts of power with a temperature rise in the motor of approximately 40.degree.C. In view of the prior art motor designs, it is an object of this invention to provide a low power synchronous motor designed to operate at a fraction of the developed torque provided by prior art motors and, in addition, to reduce substantially the current flow into the energizing winding thereby rendering the motor safe in case of accidental contact with the electrical circuit which energizes the stator windings. Further, because of the reduced power requirements, the temperature rise in the motor is reduced thereby permitting greater durability of the various motor parts.
Another object of this invention is to provide a highly efficient motor having a motor structure capable of being easily assembled by means of assembly line techniques.